In a CDMA mobile communication system, a plurality of mobile stations typically use the same frequency band to perform radio communication, and wave interference caused by the radio communication by other mobile stations, i.e., multi-user interference, is a chief cause of limitation of the subscriber capacity of a mobile communication system. To increase the subscriber capacity, the adaptive antenna technology is effective for suppressing interference waves during reception, and during transmission, for avoiding transmission in unnecessary directions and thus reducing interference power that is applied to other mobile stations.
As a prior-art example of a transceiving device that adopts the adaptive antenna technology (hereinbelow referred to as an adaptive antenna transceiving device), a technology that uses a plurality of antenna devices for controlling directivity is described in non-patent document 1 (NTT DoCoMo Technical Journal, Vol. 8, No. 1, April 2000). Non-patent document 1 discloses increasing antenna gain in the direction of a mobile station by conferring weight coefficients (reception antenna weights) to each of received signals that are received by a plurality of antenna devices during reception, and during transmission, directing the transmission beam toward the mobile station that is the object of transmission by multiplying weight coefficients (transmission antenna weights) that are generated based on the reception antenna weights by the transmission data for each antenna device.
As a method of generating the reception antenna weights, a method is described in non-patent document 2 (S. Tanaka, M. Sawahashi, and F. Adachi, “Pilot symbol-assisted decision directed coherent adaptive array diversity for DS-CDMA mobile radio reverse link,” IEICE Trans. Fundamentals, Vol. E80-A, pp. 2445-2454, December 1997) for implementing control so as to minimize the mean-squared-error between a pilot symbol after despreading and a signal after RAKE synthesis that is generated by referring to information data symbols that have been provisionally determined. In addition, non-patent document 3 (Tanaka, Harada, Ihara, Sawahashi, Adachi, “Outdoor test characteristics of adaptive antenna array diversity reception in W-CDMA,” Shingaku Gihou, RCS99-127, pp. 45-50, October 1999) describes an example of using the transmission antenna weights that are generated based on the above-described reception antenna weights in downlink transmission (transmission in the direction from the radio base station to a mobile station).
In a CDMA mobile communication system, transmission power control (hereinbelow abbreviated as TPC) is generally implemented to ensure transmission quality while avoiding unnecessary interference to other mobile station. In particular, the TPC technology is indispensable in CDMA because common frequency interference is produced by the assignment of a common frequency to a plurality of mobile stations.
The relation between TPC and a transmission beam in downlink transmission is next considered as an example.
In downlink transmission, the directivity of the transmission beam is controlled by using a plurality of antenna devices that are provided in a radio base station and multiplying the transmission antenna weights of each by transmission data. A mobile station instructs the radio base station to reduce the transmission power if the reception quality exceeds a desired value and instructs the radio base station to increase the transmission power if the reception quality falls below the desired value. The instructions to increase or decrease the transmission power (hereinbelow referred to as increase/decrease instructions) from the mobile station to the radio base station use TPC bits that are included in frames that are transmitted from the mobile station to the radio base station in each of prescribed cycles. The radio base station extracts the TPC bits from the frames that are transmitted from the mobile stations and increases or decreases the transmission power to that mobile station in accordance with the instructions.
The following explanation regards an adaptive antenna transceiving device of the prior art with reference to FIG. 1. The adaptive antenna transceiving device that is shown in FIG. 1 is an example of the configuration for executing TPC in the adaptive antenna transceiving device that is described in FIG. 1 of non-patent document 1.
As shown in FIG. 1, this adaptive antenna transceiving device of the prior art is a configuration that includes: a plurality (N, where N is a positive integer) of antenna devices 301_1-301_N that are arranged in an array; receiving-side multipliers 302_1-302_N for multiplying reception antenna weights by the received signals that have been received by antenna devices 301_1-301_N; adder 303 for adding (synthesizing) the plurality of received signals that have been multiplied by the reception antenna weights and supplying the result as reproduction data; reception antenna weight generation circuit 304 for, based on the reproduction data that have been supplied as output from adder 303, calculating the optimum reception antenna weights that are to be multiplied by the received signals that have been received by each of antenna devices 301_1-301_N and supplying the results to each of corresponding receiving-side multipliers 302_1-302_N; TPC bit decoding circuit 307 for extracting TPC bits from the reproduction data and then decoding these TPC bits to supply instructions to increase or decrease transmission power; antenna weight conversion circuit 305 for, based on the reception antenna weights that have been generated by reception antenna weight generation circuit 304, generating transmission antenna weights, and further, increasing or decreasing the transmission antenna weight in accordance with the instructions to increase or decrease transmission power that have been supplied as output from TPC bit decoding circuit 307; and transmission-side multipliers 306_1-306_N for multiplying transmission antenna weights that have been supplied as output from antenna weight conversion circuit 305 by transmission data and supplying the products to antenna devices 301_1-301_N. The adaptive antenna transceiving device that is shown in FIG. 1 shows the configuration of a baseband signal processor that performs signal processing of principally baseband transmission/reception data. The adaptive antenna transceiving device includes a radio signal transceiver (not shown) that is provided with an RF receiver for converting radio frequency signals that are received by antenna devices to baseband signals and an RF transmitter for converting baseband signals to radio frequency signals.
In this configuration, antenna weight conversion circuit 305, based on the weight coefficients (reception antenna weights) that have been generated at reception antenna weight generation circuit 304, generates transmission antenna weights for transmitting in the same direction as the directivity when receiving. In addition, antenna weight conversion circuit 305 controls the transmission power by regulating each transmission antenna weight in accordance with the instructions to increase or decrease transmission power that have been decoded at TPC bit decoding circuit 307.
Generally, reasons that can be considered for increasing the transmission power of the radio base station include cases in which the reception quality of the mobile station deteriorates due to shielding when the radio base station and mobile station are blocked by, for example, buildings, or cases in which the transmission power is increased to compensate for a drop in the reception quality of the mobile station that is the object of transmission (hereinbelow referred to as the “desired wave mobile station”) because the peak direction of the transmission beam that is formed at the radio base station diverges from the desired wave mobile station.
When the peak direction of the transmission beam diverges from the desired wave mobile station, the reception quality of the desired wave mobile station achieves the desired value through the TPC process, but when another mobile station is present in the peak direction of the transmission beam, unnecessary interference power is applied to this mobile station, and the transmission power must therefore be increased to each mobile station. The maximum transmission power of a radio base station is normally limited by the capability of the power amplifier that supplies power to the antenna devices, and an increase in the transmission power to each mobile station therefore reduces the subscriber capacity that can be accommodated by the mobile communication system.
It is an object of the present invention to provide an adaptive antenna transceiving device that can reduce unnecessary interference power that is applied to other mobile stations due to divergence of the peak direction of a transmission beam from the mobile station that is the object of transmission when applying TPC in downlink transmission, and thus, that can prevent the reduction of the subscriber capacity of a mobile communication system.